1. Field of the Invention
The present invention relates to novel method and apparatus for efficiently recovering a free acid from an acid-containing waste liquor, especially from an acid-containing waste liquor having a high acid concentration.
2. Discussion of Background
As a method for recovering an acid from an acid-containing waste liquor, diffusion dialysis employing anion exchange membranes has been proposed and has been practically employed to some extent. However, when such a method is practically put in operation, heat of diffusion is likely to accumulate, and the temperature rise takes place, thus leading to problems such as a deterioration of the ion exchange membranes and heat deformations of parts constituting the diffusion dialytic cell. Thus, it has been difficult to conduct a consistent operation for a long period of time. It has also been proposed to preliminarily cool the acid-containing waste liquor and water for recovery before their supply in order to control the temperature level within such a range that no problem will be created even when heat of dilution has accumulated in the diffusion dialytic cell. However, it has been found that the heat accumulation is unexpectedly so high that the preliminal cooling of the waste liquor or water must be great. As the result, the recovered acid also becomes cold and, it is necessary to reheat it when the acid is used at fairly high temperature. Thus, this method has been found to be ineffective from the viewpoint of the energy consumption. Further, a multistage dialytic method has been proposed as an idea to solve the problem by reducing the heat generation by dilution per unit dialytic cell. However, such a method has a problem that the installation cost increases. Thus, the commercial operation of such a method has been found to be difficult.
There has been no effective method other than the ion exchange membrane method. Thus, in spite of high concentration of a valuable acid content, the waste liquor has had to be disposed. Yet, it is required to neutralize such a waste liquor with an alkali before disposition to avoid pollution, and the cost required for such treatment has been substantial.